looks like fun,other than the heat. lol .690 balls lol you don't need a spotting scope

I like that! As well as your ingenuity on the rear sight like the others.
Slow them down and they might not cross or not as much..
I'm with you in this heat/humidity - it's not fun.
Best of luck and fun with your new ML.

Actually, I believe increasing the velocity will cause them to cross at a greater distance.

Find the load that is the most accurate for both barrels then you find your regulation point. DBML's are hard to do this because of all the variables in loading. Getting the exact same pressure on the push down, powder within a grain, nipples allowing the same amount of fire through, even slight variations in the patch lube can cause different speeds. You will have to be getting as red neck close as you can basically. And no, faster or slower will not effect regulation. They are still traveling at the same geometric line and will come to meet at the same point. Faster or slower won't affect that. Only thing speed will effect is elevation of where each barrel will hit not windage. It's getting both barrels to hit at the same point consistently that is difficult with ML's because of previously stated reasons.

The reason I believe increasing velocity will cause convergence farther from the muzzle is that increased velocity means increased recoil. With two barrels side by side and the sight in the middle, wouldn't increased recoil cause the right barrel to recoil a bit to the right and the left barrel recoil a bit to the left?

I know your ML is a shotgun and not a rifle - so who knows how or even if it was regulated to converge at one spot. To boot - you don't have any rifling.
I've owned 1/2 dozen original British BP double rifles over the years - now only 2, both cased and 1 is RB double (actually a belted ball). Whether BL or ML you can find the regulation (which is part of the fun) by bullet weight (conicals) and powder charge - yes POI will move. Slowing down will actually cause the projectile to stay in the bore longer; i.e. right bbl will go right & left to left. Faster will be the opposite.
I wrote and article for the NMLRA MuzzleBlast a few years ago, that was published Nov. 2107 titled "Three Deuces" - all 45 fast twist double rifles. Alexander Henry, John Rigby and a Whitworth.
I'll be curious to your testing as ML double rifles are nothing new - been around for 200 years +/-.

Well, it's going to be an easy test. Shoot both barrels with 65 grains of powder at 25 yards, then both with 85 grains. Then I will know the "hole" truth. :D

Dang, I'm surprised to learn that you wear underwear!!!! :)

With tiny little red and pink hearts.